**Al-Cr-N-C**

Cr44.4Al12C27.6N16 coatings have been deposited by reactive magnetron sputtering using a Cr2AlC target. The coatings showed a phase mixture of c-CrC, h-AlN and Cr with 23 GPa hardness. In addition, the structure started to change at 800 ◦C (h-Cr2AlC, Cr3C2, Cr7C3, AlN) [114]. Better coating properties than those of the (CrAlC)N coating were described for the more complex (CrAlTi)CN coating.

DC/HiPIMS sputtered Cr25Al24N51 and (CrAl)NC coatings with a Cr/Al of ca. 1 and N > C were investigated with a carbon content in the range of 1–18 at.% [115]. Nanocomposite structured (CrAl)NC/a-C coatings are formed when a critical carbon content is exceeded (a few at.%). Crystalline (CrAl)NC phases are surrounded by an amorphous carbon phase. This effect was also detected for carbon-doped AlTi-based coatings, both for sputtering [116] and arc deposition [13].

It was shown that first the hardness significantly increases from 22 to 30 GPa for a carbon content of up to about 4 at.%, e.g., Cr24Al24N48C4, then the hardness drops to about 15 GPa at a carbon content of 18 at.%. The hardness drop is enforced by two effects; by the continually growing amorphous phase—containing amorphous carbon and amorphous CrC*x*—and by the formation of hcp AlN. The carbon-doped (CrAl)NC coatings showed improved frictional characteristics in metal forming.
